The present invention relates to a fluid film bearing operating as an anti-vibration damper in a magnetic field that is concentrically oriented relative to a rotor and a stator in contact-free, relative rotation about a common longitudinal axis. The fluid film bearing of the invention may advantageously be integrated in electrodynamic and passive magnetic bearings in order to improve anti-vibration properties and to absorb loads, appearing as transient disturbances, in general, and to improve the start up operation of electrodynamic magnetic bearings, specifically. The present invention also relates to a method by which a rotor is rotationally journalled relative to a stator in a magnetic field, concentrically about a longitudinal axis.
In 1996, Lembke disclosed and claimed a homo-polar eddy-current magnetic bearing, a bearing structure which is commonly referred to the group of electrodynamic bearings (WO 98/32981). Magnetic bearings of this type provide advantages above other magnetic bearings in that the electrodynamic bearings are auto-stabilized without the need for external control electronics. More specifically, these bearings inherently provide a control current through their operation as a current generator. A comparatively high rotational speed is however required for producing the necessary generator effect, which reduces the operational output of said bearings. At lower speeds, some kind of additional start-up bearing has so far been required.
Typically, a safety bearing is arranged in most magnetic bearing applications to serve in a failure situation, and the safety bearing may preferably be used also as, the start-up bearing at low rotational speeds. A safety bearing commonly used in magnetic bearing applications is a conventional ball-bearing, supported on a rotor shaft of extended axial length in order to accommodate also the ball-bearing. However, rotor-dynamic problems arise in respect of the axial length of the rotor shaft.
Conventionally, a safety-bearing may be provided in the form of a ball-bearing having an inner diameter that is slightly over-sized relative to the outer diameter of the shaft, so that in normal operation, the shaft that is journalled on the bearing does not engage the inner diameter thereof. Such arrangement may give rise to several problems: in a situation where the shaft or rotor bears against the bearing surface, the rotor operation changes and its behavior may substantially increase the loads on the structure; another drawback of this construction is the building space that is required and which leads to an avoidable, additional axial length of the rotor.
The latter problem would be avoided if a safety bearing was integrated in the magnetic bearing, and thus avoiding the need for extra length of the rotor. Integration of a ball-bearing in the magnetic bearing is however not possible, when the air-gap available between the rotor and the stator merely represents some tenths of millimeters.
The operational effect of an aerodynamic bearing made from a thin, cornered sheet metal foil, known as an airfoil bearing, is disclosed in U.S. Pat. No. 5,427,455 (Bosley).
It is an object of the present invention to provide a fluid film bearing for a rotor in contact-free rotation relative to a stator in a magnetic field, effective to counteract oscillating rotation of the rotor.
It is another object of the present invention to provide means aid methods for avoiding the drawbacks referred to above by integrating the duplex functions of a fluid film bearing and a slide bearing in a magnetic bearing assembly.
Yet another object is to provide a bearing assembly designed for improving the start up operation of electrodynamic magnetic bearings.
A further object for the present invention is to provide a fluid film bearing that is readily integrated in electrodynamic and passive magnetic bearings in order to improve anti-vibration properties and to absorb loads appearing as transient disturbances.
These and other objects are met in a fluid film bearing and a bearing assembly as defined in the appended set of claims, wherein advantageous embodiments of the invention are further specified in the subclaims thereof.
Briefly, the present invention suggests a fluid film bearing to be realized as a sheet metal foil made from a non-magnetic and electrically conductive material. The sheet metal foil is arranged to extend axially and concentrically in an annular gap that is formed between a rotor and stator in contact-free, relative rotation about a common axis. A magnetic field is concentrically oriented relative to the same axis. Should the rotor commence to oscillate such that the sheet metal foil is displaced from it""s concentric orientation relative to the magnetic field, eddy currents will be induced in the foil which will then be effective to counteract the oscillating rotation of the rotor. The vibration reducing property also relies on a squeeze film effect generated by air or other fluid present that is captured between the sheet metal foil and the revolving periphery in relative rotation with the foil, thus eliminating the need for additional safety bearings which are commonly arranged in magnetic bearing assemblies.
In one implementation of the inventive fluid film bearing, the sheet metal foil is designed for providing slide-bearing action at low, rotational speeds, air-bearing action at higher rotational speeds and vibration reducing action at any rotational speed.